In recent years, instead of stainless materials, lightweight and strong metals each containing aluminum as the main component have been widely used as structural materials of manufacturing apparatuses for use in the fields of manufacturing electronic devices such as semiconductors and flat panel displays, and so on, i.e. vacuum thin-film forming apparatuses for use in chemical vapor deposition (CVD), physical vapor deposition (PVD), vacuum deposition, sputtering, microwave-excited plasma CVD, and so on, dry etching apparatuses for use in plasma etching, reactive ion etching (RIE), recently-developed microwave-excited plasma etching, and so on (hereinafter collectively referred to as vacuum apparatuses), cleaning apparatuses, burning apparatuses, heating apparatuses, and so on, having surfaces brought into contact with particularly corrosive fluids, radicals, or irradiated ions. In order to realize future efficient multi-kind small-quantity production, these apparatuses are each required to shift to a three-dimensional cluster tool capable of carrying out a plurality of processes for itself, each required to carry out a plurality of processes by switching the kind of gas in a single process chamber, or the like. Among practical metals, aluminum belongs to a particularly base group and, therefore, aluminum or a metal containing aluminum as the main component requires protective film formation by a proper surface treatment.
As a surface protective film when a metal containing aluminum as the main component is used as a structural material, there is conventionally known an anodized film (alumite) obtained by anodic oxidation in an electrolyte solution. If use is made as the electrolyte solution of an acid electrolyte solution (normally pH 2 or less), it is possible to form a smooth and uniform alumite coating film having a porous structure.
Further, alumite coating films are corrosion-resistant and acid electrolyte solutions are stable and easy to manage, and therefore, the alumite coating films are generally and widely used. However, an alumite coating film having a porous structure is weak against heat as a treated surface of a structural member and thus causes cracks due to a difference in thermal expansion coefficient between the aluminum base member and the alumite coating film (Patent Document 1—Japanese Unexamined Patent Application Publication (JP-A) No. H10-130884), thereby causing occurrence of particles and occurrence of corrosion and so on due to exposure of the aluminum base member.
Further, large amounts of water and so on are accumulated/adsorbed in holes of the porous structure (Patent Document 2—Japanese Examined Patent Application Publication (JP-B) No. H5-053870) and these are released in large quantities as outgas components to cause many problems such as a large reduction in the performance of a vacuum apparatus, operation failure of devices, occurrence of corrosion of the alumite coating film and the aluminum base member due to coexistence with various gases including a halogen gas and chemicals, and so on. Among halogen gases, particularly a chlorine gas is used as an etching gas in the processing, such as reactive ion etching (RIE), of a metal material and is also used in a cleaning process of a thin film forming apparatus or a dry etching apparatus and, therefore, it is important to achieve a metal surface treatment of an apparatus member that can ensure strong corrosion resistance against the chlorine gas.
In view of this, there are various proposals for alumite coating films each with a low increase rate of cracks caused by a high-temperature heat load and their forming methods. For example, there is proposed a method of forming an alumite coating film with a controlled aluminum alloy composition (Patent Document 3—Japanese Unexamined Patent Application Publication (JP-A) No. H11-181595). However, this alumite coating film also has a porous structure on the surface like the conventional one and various problems due to water remaining in holes of the porous structure remain outstanding.
Various methods are proposed for improving the problems caused by this porous structure. For example, there are proposed a sealing treatment in which an alumite coating film with a porous structure anodized in an acid electrolyte is immersed in boiling water or treated in pressurized steam, thereby forming aluminum hydroxide (boehmite layer) on the surface to fill holes (Patent Document 4—Japanese Unexamined Patent Application Publication (JP-A) No. H5-114582), a sealing treatment in a solution containing a hydrate or hydrated oxide of a metal as the main component (Patent Document 5—Japanese Unexamined Patent Application Publication (JP-A) No. 2004-060044), and so on. However, water still remains in holes of the porous structure even after the sealing treatment and the boehmite layer of aluminum hydroxide itself is also a hydrate and thus serves as a water supply source depending on the conditions such as a pressure and a temperature and, therefore, a radical solution has not yet been reached. There is also proposed a method of performing barrier-structure anodic oxidation after forming a porous-structure alumite coating film (Patent Document 6—Japanese Unexamined Patent Application Publication (JP-A) No. 2005-105300). However, since it is necessary to perform the anodic oxidation in the two processes, there is a problem that the manufacturing cost increases.
Besides, as a surface treatment when a metal containing aluminum as the main component is used as a structural member, use is made of a thermal spraying method that melts and sprays a powder material of a metal, an alloy, a ceramic, or a combination of the ceramic and the metal or the alloy (Patent Document 7—Japanese Unexamined Patent Application Publication (JP-A) No. H9-069514). However, in the surface treatment by the thermal spraying method, there remains a problem in that since it is difficult to suppress formation of pores where the film surface and the base member communicate with each other through holes, when a corrosive gas such as a halogen gas is used in an apparatus, portions of the metal, containing aluminum as the main component, of the base member, that are brought into contact with the corrosive gas through the pores, are subjected to corrosion.